1. Field of the Invention
The present invention relates to construction materials and in particular cementitious construction materials.
Each year about 250 million tires in the USA and about 1,000 million in the world are scrapped. Current trends indicate that less than 7 percent of these tires are being recycled into other products, 11 percent are being burned for energy, and 5 percent are being exported third world countries for reuse. Scrap Tire Technology and Markets, US Environmental Protection Agency, Office of Solid Waste, Washington, D.C., 1993, published by Noyes Data Corporation, Park Ridge, N.J.
Over 70 percent of scrap tires end up in overcrowded landfills, and millions more are left in empty lots and illegal tire dumps. These dumps have the potential to cause serious fire and environmental hazards. Because rubber tires do not easily decompose, economically feasible and environmentally sound alternatives for scrap tire disposal must be found.
In recent years, civil (geotechnical) engineering applications of tire shreds, which are pieces of whole tires cut into 50-300 mm pieces, have increased. The use of tire shreds as fill material in geotechnical applications has several potential benefits. In areas where underlying soil is compressible or weak, tire shreds, with their unit weight about one third of the conventional backfill, would apply a smaller overburden stress than conventional granular backfill, resulting in lower settlement and increased overall stability.
Moreover, the horizontal stress induced on retaining structural systems would be about one half lower than conventional backfill, leading to a less expensive retaining structure design.
However, the existing civil engineering applications of tire shreds face a number of technical difficulties. The quality control of the in situ compaction process of tire shreds is subject to many variables and uncertainties. Furthermore, the performance of the compacted tire shreds is highly workmanship dependent. Tire Chips-A New Road—Building Geomaterial, Humphrey, D. N., 1996, TR News, Washington, D.C., Vol. 184, No. 17.
Another potential problem of the use of tire shreds as a backfill material is the considerable amount of settlement that may be caused by surface loading. Tire Shreds as Lightweight Retaining Wall Backfill: Active Conditions, Tweedie, J. J., Humphrey, D. N., and Sanford, T. C., 1998, ASCE Journal of Geotechnical and Geoenvironmental Engineering, Vol. 124, No. 11, 1061-1070; Shredded Tires and Rubber-Sand as Lightweight Backfill, Lee, J. H., Salgado, R., Bernal, A., and Lovell, C. W., 1999, ASCE Journal of Geotechnical and Geoenvironmental Engineering, Vol.125, No.2, 132-141. Although the degree of settlement can be reduced by the appropriate mixture of soil and tire chips, the vibration loads induced on the mixture can easily cause segregation of the soil from the tire chips. Overall settlement of the fill will eventually develop under long-term conditions (Lee, et al., 1999). Furthermore, the overall unit weight of the tire chips and soil mixture is significantly increased. These factors will result in increased construction costs for a fill project.
The use of tire shreds as fill material may also be potentially subject to a process of pyrolysis. The moisture in the ground causes the steel contained in the tire shreds to corrode which, as corrosion is basically an exothermic process, leads to steady heat buildup which in turn causes an uncontrolled process of pyrolysis. The emitted gases may cause fire hazard and hydrocarbon oils may cause soil contamination. Design guidelines to minimize internal heating of tire shred fills, 1997, Ad Hoc Civ. Engrg. Com., Scrap Tire Management Counsel, Washington, D.C.; Investigation of Exothermic Reaction in Tire Shred Fill Located on SR100 in Ilwaco, Washington, Humphrey, D. N.; 1996, Report to the Federal Highway Administration, FHWA, Washington, D.C.
ASTM document D 6270-98, entitled Standard Practice for Use of Scrap Tires in Civil Engineering Applications limits the use of rubber chips containing steel wires to civil engineering applications where the fill thickness is less than 3 meters thick. For fill thicknesses of greater than 3 meters, wire-free rubber chips must be used in order to eliminate serious heating (pyrolysis) within the fill.
2. Description of the Related Art
A number of proposals have been made to make use of recycled rubber tires in construction and building materials.
U.S. Pat. No. 5,800,754 (Woods, 1998) discloses a process for forming a building unit from ground rubber tires with 15% to 20% of adhesive comprising asphalt. The mixture is then placed into a heated mound and subjected to heat and pressure to form a building block.
U.S. Pat. No. 5,425,904 (Smits, 1995) discloses a process for activating vulcanized waste rubber particles by treating the waste rubber particles with a rubber latex and curing and drying the treated waste rubber particles. Also disclosed are processes for producing a rubber-like article by molding the activated waste rubber particles while applying heat and pressure.
U.S. Pat. No. 5,316,708 (Drews, 1994) discloses a process of making building block members by mixing natural latex with shredded vehicle tires to form a mixture, placing the mixture in a mold, applying pressure to compress the mixture, and maintaining pressure for a time period which the latex hardens and cures.
U.S. Pat. No. 5,258,222 (Crivelli, 1993) discloses a process of mixing coarse rubber crumbs with coarse siliceous grains to form a closely packed mixture and wetting the surfaces of the coarse particles with a polymerizable liquid binder to provide a viscous slurry. The slurry is then cast into a sheet-like configuration, and the sheet-like configuration is used under sufficient heat and for a sufficient time to produce sheet-like products, such as: pavers, tiles, and shingles.
U.S. Pat. No. 5,094,905 (Murray, 1990) discloses a process of making structural articles from rubber tire fragments with adhesive. The tire fragments are mixed with an adhesive and molded, preferably under pressure, into a shape such as a rectangular beam. These items can be used as structural articles such as landscaping ties, dock bumpers for boat docks or truck loading docks, as resilient mats for workers or farm animals. Alternatively, it can be used as substitutes for various products that are normally made of wood but which do not need to withstand large longitudinal loads.
However, the proposals heretofore known suffer from a number of disadvantages:                The bonding agents are generally relatively expensive adhesive or latex compounds. The production costs of the resulting products are inevitably very high.        The molding processes are generally involved with heating and pressing in the mold for a substantial time period. Thus, the product and energy costs are increased substantially.        In a general sense, all the proposals heretofore resulted in the production of closely packed products for building, construction, and outdoor applications. The products thus have to resist relatively large structural loads, impact loads, and normal wear and tear as expected in most of the outdoor applications. However, the lightweight and granular natures of ground rubber crumbs are, generally not fully utilized in these proposals.        
In terms of lightweight construction technology, U.S. Pat. No. 5,785,419 (McKelvey, 1998) teaches a process for developing a lightweight building material for use in above grade construction comprising cement, fly ash, cellulose fiber (mostly from recycled paper pulp), and water.
U.S. Pat. No. 5,569,426 (Le Blanc, 1996) involves the development of a lightweight cement block by mixing a predetermined ratio of sawdust, cement, sand, and water. However, this method and that of McKelvey have the major disadvantage that the resulting material can be slowly decomposed when it is buried below grade, especially under partially saturated conditions.
U.S. Pat. No. 5,785,419 (Rodgers, 1998) proposes a method for preparing a lightweight concrete include mixing a slurry comprising water, cementing binder, fine grain aggregate and polystyrene pellets. However, the cost of production of such a lightweight material is quite high and the resulting material is impermeable with a closed-form structure.
U.S. Pat. No. 5,290,356 (Frankowski, 1994) and U.S. Pat. No. 5,456,751 (Zandi, et al, 1995) disclose processes for making concrete materials which contain particulate rubber or rubber crumbs (preferably recycled from automobile tires). The materials proposed in these documents are typically used in cement boards, rubber reinforced mortar and road surfaces. However, similar limitations as for U.S. Pat. No. 5,785,419 (Rodgers 1998) occur as the resulting materials are impermeable because they have a closed pore structure.
Thus, there remains a need to produce not only an improved construction material having beneficial mechanical properties but also for providing, at least in preferred embodiments, an environmentally sound way of disposing of scrap rubber, and in particular rubber tires.